MLZ is a cooperation between:> Technische Universität München> Helmholtz-Zentrum Geesthacht> Forschungszentrum Jülich
The magnetic (H, T) phase diagrams of the frustrated 2D antiferromagnetic LiMPO4 (M = Ni, Co, Fe, Mn) systems are rich and surprisingly similar. The prime example is XY-like LiNiPO4, exhibiting 7 magnetic phases below 55 T longitudinal field. Using epithermal neutrons with pulsed magnetic fields, we successfully probed the magnetic structures in LiNiPO4 in longitudinal fields up to 42 T. At 19 T, we find a counterintuitive phase transition between an IC spiral structure – consistent with XY-anisotropy – and a re-entrant magnetoelectric (ME) commensurate order polarized along the applied field. This magnetized variant of the zero field structure is confirmed by a microscopic model quantitatively describing both the ME response between 19-21 T and that in zero field. In the sister compound LiFePO4, we recently discovered weak canting components in the magnetic structure in zero field. These indicate that the crystal symmetry is lower than previously established.
Through the Dzyaloshinky-Moriya interaction, this canting is shown to give rise to a ME response that describes the temperature dependency of the ME coupling strength. It can generally be said that the single anisotropy terms of the LiMPO4 systems are surprisingly strong, but with similar exchange constants. This makes them ideal for synthesizing mixed anisotropy magnets – where exchange interactions and single ion anisotropies compete. In the case of the (Co, Ni) substitution, this leads to a field induced phase transition from long to short range order. Substituting Fe for Ni on the other hand, gives rise to an additional spontaneous ME order parameter and offers the possibility of tailoring the magnetoelectric response.
|Uhrzeit||14:30 - 15:30 Uhr|
|Sprecher||Rasmus Toft-Petersen (Technical University of Denmark, Department of Physics)|